Hot answers tagged riichi-mahjong
I think most "official" rules will not allow you to do this. (For reference, I looked up the Japanese Mahjong rules from the European Mahjong association here.) In principle, I think swapping tiles that do not change the wait structure would be allowed, but it is impossible to verify. In your example of swapping the 5 for the 8, all that is visible is the ...
According to the Japanese Wikipedia article on riichi: 打牌の選択（手牌の入れ替え）ができない。つまり、和了する場合と後述の暗槓する場合を除いて、自摸した牌をそのまま捨てなければならない。 Translating, this means "You are no longer allowed to select your discards (switching the drawn tile with a tile in your hand). In other words, save for drawing your winning tile or calling a concealed kan as described below, you ...
You replace them, because otherwise we will end up with five copies of the 5-numbered tiles, which is not the intention of using red fives.
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